1. Field of the Invention
The present invention relates to wireless communication systems, and more particularly, to cellular telephone electronics for use in a dual band cellular system.
2. Description of the Related Art
A cellular telephone includes a radio frequency (RF) section and a baseband controller. The RF section up-converts a modulated intermediate frequency (IF) signal from a modem unit into RF energy in a transmission mode, and down-converts a received RF signal into an IF signal in a reception mode. Such an RF section generally includes a plurality of mixers, automatic gain controllers, filters, local oscillators, and amplifiers.
In a cellular system, the radio frequency used for transmitting is usually different from the frequency used for reception. Each system has various methods for discriminating the frequencies. The method for selecting the intermediate frequencies during the transmission and reception modes is influenced by many different factors. To simplify the hardware and lower the cost, the intermediate frequencies during transmission and reception should be selected such that a separation of the intermediate frequencies accurately coincides with a separation of the radio frequencies. In this case, it is possible to use a single local oscillator for both the transmission and reception chains. Other components of the cellular telephone include an IF transmission chain, an IF reception chain, an RF transmission chain, and an RF reception chain.
Referring to FIG. 1, a conventional single band cellular telephone includes a modem section and an RF section. The modem section consists of a modem unit 100. The RF section includes an antenna 101, a duplexer 102 for duplexing transmission and reception signals, a low noise amplifier (LNA) 103, an RF reception bandpass filter 105, a power amplifier 104, an RF transmission bandpass filter 106, mixers 107 and 108, a local oscillator 109 for the reception chain, an IF reception bandpass filter 110, an IF reception automatic gain controller 112, an IF transmission bandpass filter 111, and a selective variable gain amplifier 113. In the system, the single local oscillator can be used for both the transmit and receive chains.
Prior art cellular telephones capable of use in dual band systems typically utilize three local oscillators. A first local oscillator is used for both transmit and receive when operating in a first of the two bands, similar to the configuration of FIG. 1. Second and third local oscillators are used for the transmit and receive paths, respectively, when operating in the second band. The three local oscillators are employed in this manner in order to maintain the same frequency separation between the transmit and receive IF frequencies, regardless of which operating band is being used.